Trailing wire antenna

ABSTRACT

An improved trailing wire antenna system adapted to be trailed behind a flying aircraft and whose radiation is substantially completely vertically polarized rather than horizontally polarized. The antenna system consists essentially of two parallel antenna conductors which form a resonant transmission line, the horizontal radiation from which is substantially suppressed so that most of the radiation is vertically polarized. The antenna system is particularly useful in the low and very low frequency ranges.

United States Patent [191 Karaganis et al.

[451 Aug. 13, 1974 TRAILING WIRE ANTENNA [75] Inventors: James J. Karaganis, Rockville; Peter R. Payne, Silver Spring, both of Md.

[73] Assignee: Wyle Laboratories, El Segundo,

PrimaryExaminerl-lerman Karl Saalbach Assistant E.mminer-Marvin Nussbaum Attorney, Agent, or FirmSughrue, Rothwell, Mion, Zinn & Macpeak 5 7 'ABSTRACT An improved trailing wire antenna system adapted to be trailed behind a flying aircraft and whose radiation is substantially completely vertically polarized rather than horizontally polarized. The antenna system consists essentially of two parallel antenna conductors which form a resonant transmission line, the horizontal radiation from which is substantially suppressed so that most of the radiation is vertically polarized. The antenna system is particularly useful in the low and very low frequency ranges.

15 Claims, 14 Drawing Figures ATENTEDmc 1 a mu 3; 82 9;861 sum 18? 2 HG. |C| PRIOR ART A FLN 4 32 xMTRE z I 44 W as h FIG. 20

FIG. 2b

I if \1 mvmozzs JAMES J. KARAGANIS FIG. 3 PETER R. PAYNE 4 4 mw we ATTORNEYS 1 I TRAILING WIRE ANTENNA BACKGROUND OF THE INVENTION to obtain a greater amount of vertically polarized radiation. A vertically polarized component of radiation is required since horizontally polarized radiation experiences far greater attenuation as a ground wave than vertically polarized radiation. However, even with the use of extremely heavy masses, only a few hundred feet at the end of the wire will assume an approximately vertical position when the wire is 30,000 to 50,000 feet long. Consequently, typically only about 2.5 per cent of the radiation would be vertically polarized, and this was due predominantly to the slight depression of the bulk of the antenna from the horizontal and not to the relatively short vertical end portion of the wire.

The wire had to be in the range of 30,000 to 50,000 feet, i.e. close to /z wavelength, or even longer in the very low frequency ranges in order to make the antenna resonant. Even at higher frequencies, the antenna had to be at least a quarter of a wavelength long. However, at lower frequencies, because of the large negative reactance of the aircraft, the length of the wire would have to approach one-half of the wavelength of the transmission frequency.

SUMMARY OF THE INVENTION The invention may be summarized as an improved trailing wire antenna system for use with an aircraft and comprising two vertically spaced horizontal conductor portions which form a resonant line. The horizontally polarized radiation from the system is substantially suppressed so that most of the far field radiation is vertically polarized. The horizontal length of the conductor portions need not be greater than approximately onequarter of a wavelength and in some embodiments of the invention may be less. The vertical polarization power efficiency of the improved antenna of this invention is theoretically 100 percent thereby representing a net gain of almost sixteen decibels over the prior art trailing wire antenna.

BRIEF DESCRIPTION OF THE DRAWING FIGS. la and lb are schematic diagrams illustrating the prior art single wire trailing antenna;

FIG. 2a is a schematic diagram of a preferred embodiment of the improved trailing wire antenna system of this invention;

FIG. 2b is a simplified equivalent circuit diagram of the antenna system of FIG. 2a;

FIG. 3 is a schematic diagram of a modified form of an antenna conductor; and

FIGS. 4 through 12 are schematic diagrams illustrating other embodiments of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIGS. la and lb illustrate a conventional prior art trailing wire antenna. A single wire 10 is fixed at one end to an aircraft 12. A mass 14 is attached to the free end of the wire 10 so that the wire forms an angle 5 with the horizontal 16. The aircraft is assumed to be flying in a horizontal plane.

FIG. lb illustrates a simplified equivalent circuit diagram of the antenna and plane illustrated in FIG. la. In

FIG. 1b, the dimension L represents the length of aircraft l2, dimension L is the length of conductorportion 16 which represents the relatively long portion of wire 10 which makes an angle (1) with the horizontal, and dimension L is the length of the conductor portion 18 which represents the substantially vertical portion of the free end of the wire 10in FIG. la. An AC generator 20 represents a transmitter located in aircraft 12 and driving the antenna 10. The dashed line segments 22, 24 and 26 represent the approximate distribution of the current I along the antenna system formed by the aircraft l2, conductor portion 16 and conductor portion 18, respectively. The radiation from the aircraft and the conductor portion 16 is substantially horizontally polarized and the radiation from conductor portion 18 is substantially vertically polarized.

As a practical matter, for a wire 10 having a length of 30,000 to 50,000 feet, the conductor portion 18 is only a few hundred feet long. It can be shown that the vertical polarization from this portion is so small that it is practically negligible. The only useful vertical polarization from such an antenna system is derived from the vertical component of the long conductor portion 16 of wire 10. The ratio of the broadside vertically polarized radiation to the total broadside radiation from conductor portion 16 is equal to sin%. For a typical angle d) of 9, therefore, only about 2.5 per cent of the total power radiated from conductor 16 along the length L is vertically polarized, and therefore only that small percentage can be considered effective for long range communication. In other words, the net polarization insertion loss in the length L of the conductor portion 16 is about 16 decibels. The vertical polarization from the vertical conductor portion 18 becomes sub stantial only when this portion approaches a quarter wavelength. Since at low and very low frequencies, such a length of vertical portion 18 cannot, as a practical matter, be obtained by increasing the weight of mass 14, it can be seen that the percentage of useful vertical polarization is almost completely dependent upon the angle (1).

In such a prior art antenna, the aircraft l2 acts as a driven element for the antenna 10. If we assume an aircraft with effective length of 150 feet and an effective diameter of l5 feet, the negative reactance of the aircraft for various frequencies can be calculated to obtain the following values: 67 ohms at l mHz, 1,250 ohms at kHz and 12,500 ohms at l0 kHz. To compensate for this negative reactance at l mI-Iz, the trailing wire 10 need be only slightly more than a quarter wavelength. However, at 100 kHz, the trailing wire must approach /2 wavelength. In other words, below medium frequencies, the antenna lengths must approach wavelength to achieve reasonable reactance values at the feed point of the antenna. However, as the length of the trailing wire 10 approaches the half wavelength, the antenna resistance reaches high unwieldly values. Therefore, for reasonable power transfer at very low frequencies, enormous matching components would be required in the aircraft.

In the preferred embodiment of the invention illustrated in FIG. 2a, the reactance of the airplane can be neglected. This embodiment consists essentially of a horizontal wire 30 connected to one terminal of the transmitter 32 in aircraft 34. A substantially vertical wire 36 has one end attached to the other terminal of transmitter 32 and the other end fixed to a heavy mass 38 which in practice may be a bomb of suitable weight. Extending from the bomb 38 rearward of plane 34 is another horizontal wire 40 which is electrically connected to wire 36 so that wires 36 and 40 may be considered as one wire electrically. Mechanically, wire 36 is operated by a winch in the aircraft and supports the bomb, and the bomb contains a second winch (not shown) which may be electrically operated to reel out and in the wire 40. Alternatively, the bomb may be supported by a separate cable. In this case, the second winch is also located in the aircraft, and only a pulley is required in the bomb. The antenna portions corresponding to conductors 36 and 40 would then be formed by a single wire extending from the second winch, around the pulley, and then to the rear of the aircraft. Drogues 42 and 43 are connected to the ends of wires 30 and 40, respectively, in order to maintain them in a substantially horizontal position. Wires 30 and 40 are each substantially a quarter wavelength long so that the wires 30 and 40 form a tuned parallel conductor transmission line at the transmission frequency of the transmitter. Electrically, the resulting antenna system may also be considered as an open circuited stub line.

With such an arrangement, the far field radiations of horizontal conductors 30 and 40 effectively cancel so that there is relatively little horizontally polarized radiation from the antenna system formed by the wires 30, 36 and 40. The ends of the horizontal wires 30 and 40 nearest the airplane form with vertical conductor 36 a current dipole which radiates vertically polarized radiatron.

The equivalent circuit diagram in FIG. 2b illustrates the direction in which the antenna. current I flows in the antenna system. Resistor R represents the radiation resistance of thecurrent dipole, and resistance R represents the radiation resistance due to the horizontal conductors and to the mutual or cross effect between the horizontal and vertical conductors. Since I flows in opposite directions in conductors 30 and 40, their far field radiations tend to cancel. The closer the conductors, the greater the cancellation. When the conductor lengths approach twenty times the vertical spacing of the conductors and the vertical spacing is less than onetenth of a wavelength, then conductors 30 and 40 form a tuned'open-circuited transmission line and with vertical wire 36 form acomplete end-loaded stub. These dimensions would be satisfied at very low carrier or operating frequencies, such as 5 kHz. At higher frequencies, such as kHz, with the same length of vertical conductor 36, the length-to-spacing ratio decreases so there is less cancellation of horizontally polarized radiation but the larger proportion of vertical conductor produces a greater amount of vertically polarized radiation at the higher frequency.

In a typical antenna system as illustrated in FIG. 2a for a kw output, the horizontal conductors and are each 56 inch cable and the .vertical conductor 36 is a 1 inch cable. The mass 38 weighs 3,000 pounds and the length of conductor 36 is 2,000 feet. For a transmisnum mesh sock. In fact, the conductors 30, 36 and 40 may take many forms which have advantages over solid metal cables. The hollow mesh may be made of nonconducting material, such as fiberglas, and a metal fiber may be woven in a helical path along the mesh sock. Such an arrangement provides a longer conductive path, i.e. longer electical length, with a shorter overall length of the sock. The ptich of the helix may be made uniform along its entire length in order to reduce the antenna length at all frequencies. However, in an important variation, the helix pitch may be made shorter near the winch end of the antenna as more of the antenna is reeled out from the aircraft. That is, the helix on the antenna would be uniform at higher frequencies, but for lower frequencies, where more antenna length is required to obtain an electrical quarter wavelength, the shorter helical pitch reduces the physical length of antenna required. Furthermore, instead of a mesh, a helical conductor may be wound or coated on a solid non-conducting cable made of fiberglas, for example. This variation is shown in FIG. 3 where a nonconducting carrier 46, such as a mesh or solid cable, carries a helical conductor 47 whose pitch near the free end 48 is uniform, but has been reduced near the winch 49.

FIGS. 4-11 illustrate variations of the basic resonant line trailing antenna system illustrated in FIG. 2a. In each of these figures, the transmitter in the aircraft is represented by a generator 50 and the plane is assumed to be traveling in the direction indicated by the arrow 52. The drogues and weights necessary to form the antenna geometry in each figure are not shown, but the proper placement of drogues and weights will be obvious to one skilled in art.

In FIG. 4 there is shown a shunt feed system which permits a better impedance match to be obtained. Horizontal wires 54 and 56 correspond to the wires 30 and 40 in FIG. 2a and form with the vertical conductor 58 the basic resonant line antenna system. Power from the transmitter 50 is fed via a small transmission line 60 to the points 62 and 64 on the horizontal antenna wires 54 and 56. Line 60 may be either a twin conductor transmission line or a coaxial cable. The points 62 and 64 are chosen such that the antenna impedance at those points is equal to a desired value, such as the characteristic impedance of the transmission line 60. Points 62 and 64 are substantially opposite each other in this symmetrical feed arrangement. A typical impedance of a coaxial transmission line 60 would be ohms, whereas the impedance of the asymmetrical series fed embodiment of FIG. 2a might be on the order of 2 ohms.

FIG. 5 illustrates an impedance matched antenna in which the displaced feed points are effectively in series with the horizontal elements shown in FIG. 4. Here, the individual conductors of the transmission line 60 are connected in series with horizontal conductors 65 and 66. Conductors 67 and 68 extend along conductors 65 and 66 to the points 69 and 70 which are chosen such that the resistance of the antenna system equals a desired value, such as the characteristic impedance of the transmission line 60. Points 69 and 70 are substantially opposite each other in this symmetrical feed arrangement.

FIGS. 6 and 7 are variations of FIGS. 4 and 5, respectively, in which an asymmetrical feed at the upper left corner of the antenna system is substituted for the symmetrical feed by a transmission line from the generator to the electrical center of the antenna as illustrated in FIGS. 4 and 5. Here the points 71, 72 and 73, 74 are not opposite each other.

FIG. 8 illustrates a variation of FIG. 2a in which the physical length of the antenna system may be actually less than a quarter wavelength. To accomplish this result, horizontal conductors 80 and 82 are each endloaded with lump capacitances in the form of large cylindrical metallic conductors 84 and 86, so that the conductors still form an electrically resonant or tuned line even though they are physically shorter than a quarter wavelength.

FIG. 9 illustrates a non-uniform impedance variation of FIG. 2a in which the physical length of the antenna system is less than one-quarter wavelength. Vertical conductor 88 and lower horizontal conductor 90 are positioned similarly to conductors 36 and 40 illustrated in FIG. 2a. However, the upper conductor 92 is fixed to another weight or bomb 94 at the point 96 so that the conductors 92 and 90 are physically much closer from the point 96 to their free ends, thereby effectively loading the ends of the antenna conductors so that the antenna system is maintained electrically resonant even though the physical length of the conductor is less than one-quarter wavelength.

FIG. illustrates another variation of FIG. 2a in which horizontal conductors 98 and 100 and a vertical conductor 102 effectively form a shorted quarter-wave transmission line which presents a high resistance load to generator 40 rather than a low resistance which is available with the open circuit embodiment illustrated in FIG. 2a. The series and shunt feed impedance matching arrangements shown in FIGS. 4-7 may also be applied to this variation.

FIG. 11 shows a re-entrant transmission line variation of FIG. 2a in which the overall length of the antenna system need be only one-eighth wavelength long. Here the upper horizontal conductor 104 extends horizontally for approximately /a wavelength and then is folded back to form a substantially vertical conductor portion 106 and another horizontal portion 108. The end 110 of conductor portion 108 is supported by a nonconducting cable 112 from the aircraft in which the transmitter 114 is located. Cable 112 may be made of fiberglas or nylon, for example. Another line 116 extends from the other terminal of transmitter 50 parallel to conductor 104 for a distance of /a wavelength. This re-entrant version of the basic antenna system of FIG. 2a exploits the inherent capacity of the aircraft as additional end-loading.

FIG. '12 illustrates a variation of FIG. 11 in which the non-uniform impedance line approach is used in a manner similar to that illustrated in FIG. 8 to further reduce the physical length of the horizontal conductors which is required to achieve electrical resonance.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

l. A trailing wire antenna system for use in combination with a transmitter in a flying aircraft comprising:

a. first conductor means adapted to be coupled to one output terminal of the transmitter and adapted to be trailed behind said aircraft in a substantially horizontal position,

b. second conductor means adapted to be coupled to the other output terminal of said transmitter and adapted to be trailed behind said aircraft in a substantially horizontal position but vertically spaced from said first conductor means, and

c. said first and second conductor means forming a resonant line at the operating frequency of said transmitter, whereby the horizontally polarized radiation field of said antenna system is substantially suppressed so that substantially large amounts of the radiation field of said antenna are vertically polarized.

2. A trailing wire antenna system as defined in claim 1 wherein said first and second conductor means comprise first and second horizontal conductors, respectively, each one-quarter wavelength long at said operating frequency. 1

3. A trailing wire antenna system as defined in claim 2 wherein said second conductor means further comprises a substantially vertical conductor connected at one end to said second horizontal wire and adapted to be connected at its other end to said other output terminal of said transmitter.

4. A trailing wire antenna system as defined in claim 3 wherein the trailing ends of said horizontal conductors are open-circuited, thereby forming a quarterwave open-circuited transmission line.

5. A trailing wire antenna system as defined in claim 3 wherein the trailing ends of said horizontal conductors are short-circuited, thereby forming a quarterwave short-circuited transmission line.

6. A trailing wire antenna system as defined in claim 3 wherein at least one of said conductors comprises a hollow cylinder made of metallic mesh.

7. A trailing wire antenna system as defined in claim 3 wherein at least one of the conductors comprises a hollow non-conducting mesh sock and a conductive material woven in a helix along the length of the sock.

8. A trailing wire antenna system as defined in claim 7 wherein the pitch of said helix on the portion of the sock nearer the aircraft is shorter than the pitch of the helix on the trailing portion of the sock.

9. A trailing wire antenna system as defined in claim 3 wherein at least one of said conductors comprises a solid non-conducting cable carrying a conductive material forming a helix-along the length of the cable.

10. A trailing wire antenna system as defined in claim 9 wherein the pitch of said helix on the cable end nearest the aircraft is shorter than the pitch of said helix on the trailing end of said cable.

11. A trailing wire antenna system as defined inclaim 1 further comprising a transmission line adapted to be coupled at one end thereof to a selected feed point on each of said conductors and adapted to be coupled at the other end thereof to the terminals of said transmitter, each feed point being selected in accordance with a desired antenna impedance.

12. A trailing wire antenna system as defined in claim 3 further comprising means for end-loading the trailing end of each horizontal conductor, thereby obtaining electrical resonance .with horizontal conductors less a. first conductor means adapted to be connected to one output terminal of said transmitter and adapted to be trailed behind said aircraft so that it has a horizontal length substantially one-eighth wavelength long and a drooping vertical portion at the trailing end thereof,

b. second conductor means coupled at one end thereof to said vertical portion of said first conductor means and extending horizontally for approximately one-eighth of a wavelength toward said aircraft, said second conductor means being vertically spaced relatively far from said first conductor means, and

c. a feed conductor adapted to be connected to the other terminal of said transmitter and extending along said first conductor means for approximately oneeighth wavelength and being relatively close to said first conductor'means, thereby forming a radiating resonant re-entrant line whose horizontally polarized radiation is substantially suppressed so that substantially all of the radiation from said line is vertically polarized. 

1. A trailing wire antenna system for use in combination with a transmitter in a flying aircraft comprising: a. first conductor means adapted to be coupled to one output terminal of the transmitter and adapted to be trailed behind said aircraft in a substantially horizontal position, b. second conductor means adapted to be coupled to the other output terminal of said transmitter and adapted to be trailed behind said aircraft in a substantially horizontal position but vertically spaced from said first conductor means, and c. said first and second conductor means forming a resonant line at the operating frequency of said transmitter, whereby the horizontally polarized radiation field of said antenna system is substantially suppressed so that substantially large amounts of the radiation field of said antenna are vertically polarized.
 2. A trailing wire antenna system as defined in claim 1 wherein said first and second conductor means comprise first and second horizontal conductors, respectively, each one-quarter wavelength long at said operating frequency.
 3. A trailing wire antenna system as defined in claim 2 wherein said second conductor means further comprises a substantially vertical conductor connected at one end to saId second horizontal wire and adapted to be connected at its other end to said other output terminal of said transmitter.
 4. A trailing wire antenna system as defined in claim 3 wherein the trailing ends of said horizontal conductors are open-circuited, thereby forming a quarter-wave open-circuited transmission line.
 5. A trailing wire antenna system as defined in claim 3 wherein the trailing ends of said horizontal conductors are short-circuited, thereby forming a quarter-wave short-circuited transmission line.
 6. A trailing wire antenna system as defined in claim 3 wherein at least one of said conductors comprises a hollow cylinder made of metallic mesh.
 7. A trailing wire antenna system as defined in claim 3 wherein at least one of the conductors comprises a hollow non-conducting mesh sock and a conductive material woven in a helix along the length of the sock.
 8. A trailing wire antenna system as defined in claim 7 wherein the pitch of said helix on the portion of the sock nearer the aircraft is shorter than the pitch of the helix on the trailing portion of the sock.
 9. A trailing wire antenna system as defined in claim 3 wherein at least one of said conductors comprises a solid non-conducting cable carrying a conductive material forming a helix along the length of the cable.
 10. A trailing wire antenna system as defined in claim 9 wherein the pitch of said helix on the cable end nearest the aircraft is shorter than the pitch of said helix on the trailing end of said cable.
 11. A trailing wire antenna system as defined in claim 1 further comprising a transmission line adapted to be coupled at one end thereof to a selected feed point on each of said conductors and adapted to be coupled at the other end thereof to the terminals of said transmitter, each feed point being selected in accordance with a desired antenna impedance.
 12. A trailing wire antenna system as defined in claim 3 further comprising means for end-loading the trailing end of each horizontal conductor, thereby obtaining electrical resonance with horizontal conductors less than one-quarter wavelength long.
 13. A trailing wire antenna system as defined in claim 12 wherein said end-loading means comprises a metallic sock fixed to the trailing end of each of said horizontal conductors.
 14. A trailing wire antenna system as defined in claim 12 wherein said end-loading means comprises means for reducing the spacing of said horizontal conductors near the trailing ends thereof.
 15. A trailing wire antenna system for use in combination with a transmitter in a flying aircraft comprising: a. first conductor means adapted to be connected to one output terminal of said transmitter and adapted to be trailed behind said aircraft so that it has a horizontal length substantially one-eighth wavelength long and a drooping vertical portion at the trailing end thereof, b. second conductor means coupled at one end thereof to said vertical portion of said first conductor means and extending horizontally for approximately one-eighth of a wavelength toward said aircraft, said second conductor means being vertically spaced relatively far from said first conductor means, and c. a feed conductor adapted to be connected to the other terminal of said transmitter and extending along said first conductor means for approximately one-eighth wavelength and being relatively close to said first conductor means, thereby forming a radiating resonant re-entrant line whose horizontally polarized radiation is substantially suppressed so that substantially all of the radiation from said line is vertically polarized. 